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Hp Drawing. Original No. 1,836,448, dated December 15, 1931, Serial No. 483,763, filed September 22,

Reis sued Feb. 7, 1933 UNITED sTArE's PATENT OFFICE EDWIN W. COLT, OF GLEN ELLYN, ILLINOIS, ASSIGNOR, IBY MESNE ASSIGNMENTS, TO

ARMOUR AND COMPANY OF DELAWARE, OF CHICAGO, ILLINOIS, A CORPORATION OF DELAWARE HEAT TREATMENT OF GARNET ABRASIVE 1930. Application for reissue filed January 25, 1932. Serial No. 588,807.

of crushed abrasives, including garnet,.may

be improved and the bonding action of the glue or other adhesive may be enhanced by heat treatment at low temperatures of from 200 to 350 C., whereby to drive off the moisture and other surface impurities, this heat treatment being at temperatures not sufliciently high to affect the grains as to their chemical or physical characteristics.

Almandite and rhodolite, those varieties of garnet most available for commercial use as an abrasive, contain a considerable percentage of iron in aferrous form and a smaller percentage of iron in the ferric form. These percentages are not affected by known heat treatment at moderate temperatures, that is to say sufiicient to drive off surface moisture and eliminate adhering combustibleimpurities.

I have discovered, however, that if the crushed garnet be roasted in a suitable furnace at temperatures of from 400 to 950 C. for a period of from four to sixteen hours, the physical and chemical properties of the garnet are changed in an unexpected manner and to a remarkable degree.

The fracture is improved, the grains change in color to a deeper red, glisten as if wet, assume metallic luster, become harder, tougher and sharper, the abrasive qualities being greatly improved.

The term hardness is employed herein and in the appended claims, as applied to abrasives, to denote resistance both to wear and fracture; and the term sharpness is employed to denote the maintenance of a sharp edge due to the hardness of the abrasive.

An analysis indicates that, as compared with the untreated garnet or garnet treated at a low temperature to improve capillarity, the new product has some of the iron in the ferrous form converted into the ferric form;

that is, the ferric content is increased at the expense of the ferrous content.

Temperatures and time of treatment may appears to have a detrimental reactionary effect, the material tending to lose its crystalline structure and to break down, approaching an amorphous condition.

Exhaustive work tests demonstrate that roasted garnet fabric, paper or cloth, is as su-' perior to unroasted garnet fabric as the latter is to ordinary sandpaper, whether applied in a closed coat or in an open coat. A given sample of garnet fabric, the coating of which has been roasted, whether used on a belt or drum sander or otherwise, shows a capacity to abrade approximately twice the area as untreated garnet. The retention'of the grains by the bond is further improved, the hardness and sharpness of the grains and conse-.

quent cutting efiiciency is increased, and the useful life of the garnet fabricin service is prolonged.

I claim:

1. As an abrasive, crushed garnet roasted at sufliciently high temperature over a sufficiently extended period of time to convert a portion of the iron in ferrous form into the ferric form, said roasted garnet being characterized by a deeper red color and having an increased hardness.

2. Method of producing an abrasive, which comprises roasting garnet at a temperature sufiiciently high and for a period of time sufliciently long to increase the ferric content of the garnet, said temperature and time being maintained sufiiciently low and short to preserve the crystalline structure of the garnet.

' 3. As an abrasive, crushed garnet roasted at a temperature of from 400 C. to 950 C. for a period of time varying from four to sixteen hours, said garnet being characterized by a darker color, and a. harder and sharper ain than ordinary garnet.

4. The method of treating garnet to im* prove the hardness and sharpness of the grain, which comprises roasting crushed garnet grains for a period of from four to sixteen hours and at temperatures ranging from 400 C. to 950 C.

5. The method of producing an abrasive which comprises roasting crushed garnet grains for a period of from four to sixteen hours and at a temperature of 400 C. to 950 C. e

6. As an abrasive, crushed garnet roasted at a temperature of from 400 C. to 950 C. for a period of time varying from four to sixteen hours, said garnet being characterized by a darker color, and having an increased hardness and a higher ferric content than the unroasted garnet.

7. The method of producing a garnet abrasive, which comprises roasting the garnet at a temperature sufliciently higher than that required to improve the capillarity of the garnet and drive ofi? moisture and other surface impurities and for a time sufficiently long to materially improve the hardness of the garnet, said temperature being below that at which the crystalline structure of the garnet is destroyed.

8. The method of producing a garnet abrasive, which comprises roasting garnet at a temperature between 400 C. and 950 C. and in excess of the temperature required to improve the capillarity and drive off moisture and other surface impurities, and for a time sufliciently longer than that required to improve the capillarity whereby to materially improve the hardness of the garnet.

9. The method of producing a garnet abrasive, which comprises roasting garnet at a temperature between 400 C. and 950 C. and for a period of time suificiently long to -materially improve the hardness of the garnet.

In testimony whereof I have hereunto subscribed my name.

EDWIN W. COLT 

